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Tropical Depression Four arrives

By: Dr. Jeff Masters, 9:10 PM GMT on August 21, 2006

The strong tropical wave that moved off the coast of Africa Sunday has become large and well-organized enough to be classified as Tropical Depression Four. The depression is located southeast of the Cape Verde Islands off the coast of Africa, and is expected to track northwestwards just to the south of these islands over the next two days. The waters under the wave are 27-28 C, which is warm enough to allow some modest intensification. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, but this is far enough away that it shouldn't inhibit intensification over the next day or two, and this depression should be Tropical Debby by Tuesday afternoon. The GFDL intensifies the storm up to a strong Category 1 hurricane by Sunday, and the official NHC forecast has it close to hurricane strength by Saturday.

Unless you live in the Cape Verde Islands, this storm is unlikely to affect you. As we can see from the plot of historical storm tracks of August tropical depressions that formed near the location of the current storm (Figure 1), none of these storms have affected any land areas other than the Cape Verde Islands. The GFS model has the storm recurving northwards well east of Bermuda early next week. The Cape Verde Islands, however, are under a Tropical Storm Warning, and will get some heavy rain and high winds from this system starting Tuesday morning.

Figure 1. Historical tracks of August tropical depressions that tracked near the current position of TD 4.

Sea surface temperatures cool to below the 26.5 C threshold tropical systems prefer north of the Cape Verdes Islands, so once the system moves north of about 15 N latitude, it may weaken a bit before it crosses back over warmer waters later in the week. The dry air and Saharan dust to the north of the Cape Verde Islands may cause some trouble for the storm later this week.

Figure 2. Visible satellite image of TD 4 at 1:45pm EDT Mon Aug 21 2006. Image credit: Navy Research Lab.

Elsewhere in the tropical Atlantic
An area of thunderstorms over the Bahamas is associated with an upper level low pressure system, and development is not expected. An area of thunderstorms west of Jamaica is drifting northwards towards Cuba and South Florida, and is associated with a tropical wave. Upper level winds are not favorable for development of this area, either. The long-range GFS model is predicting a series of 2 or 3 more tropical waves will emerge off the coast of Africa over the next two weeks and intensify into tropical storms. Each of these storms eventually recurves out to sea without affecting any land areas. It is possible to have a very active hurricane season and have all (or nearly all) of the storms miss land!

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.